The present invention relates to an acceleration sensor of bimorph cell structure and an acceleration device using the acceleration sensor.
The acceleration sensor, as well as the acceleration device using the acceleration sensor, are used for the purpose of automotive posture control, seismic detection, etc. What is specifically required in the recent application fields is that which is capable of detecting low frequency acceleration with high accuracy and high reliability.
FIG. 22 is a cross sectional view of a conventional acceleration device. An acceleration sensor 5 used in the device is formed of a pair of rectangular piezoelectric ceramic plates 3, 4 bonded together by an adhesive with respective polarization axes reversed to each other, and external electrodes 6 and 7 provided on respective outer surfaces of the piezoelectric ceramic plates 3, 4. One end of the acceleration sensor 5 is fixed on a protruding section 2 of a board 1 with a conductive adhesive, and the external electrode 6 and a signal outlet cable 10 are connected by means of a wire 8. The acceleration sensor 5 is covered by a cap 9 for protection.
When acceleration is effected, the acceleration sensor 5 vibrates in the direction of thickness and generates electric charges corresponding to the size of displacement. The acceleration is known by detecting the quantity of the electric charge. The detection sensitivity of acceleration is in proportion to the quantity of the electric charge. The sensitivity is proportional to the length of piezoelectric ceramic plate 3,4, from the fixed end to the tip end of vibrating portion, or the length of free vibration section L2. Therefore, the sensitivity may be raised by increasing the length L2. This, however, lowers the resonance frequency; as a result, the highest detectable frequency becomes low. On the other hand, a shorter L2 raises the upper limit of detectable frequency, but it lowers the detection sensitivity.
A conventional acceleration device for the low frequency detection is provided with a field effect transistor and a resistor connected to the acceleration sensor 5, for making the output impedance small. And a low-cut filter is formed by the piezoelectric ceramic plate 3, 4 and the resistor. The cutoff frequency f is determined by the following formula:
f=1/2xcfx80RC
where C is capacitance of the piezoelectric ceramic plate 3, 4, and R is resistance value of the resistor. Accordingly, in order to detect the low frequency, the electrostatic capacitance C of the piezoelectric plate 3, 4, or the resistance value R, have to be made larger.
However, the use of a large R resistor easily leads to a noise absorption; as a result, the detection of low frequency acceleration turns out to be difficult. Therefore, it is essential to make the piezoelectric ceramic plate of great capacitance C readily available.
In conventional acceleration devices, because the acceleration sensor 5 has been fixed at one end on the protruding section 2 using a conductive adhesive, it has been difficult to keep the state of fixing, which is relevant to detection characteristics, under strict control, and variations in the length L2 have also been large. This means that there is a wide variance in the detection sensitivity.
One of the objectives of the present invention is to offer an acceleration sensor and an acceleration device that have an improved detection characteristic in the low frequency.
Another objective of the present invention is to offer an acceleration sensor and an acceleration device that have a well-controlled variance in the detection sensitivity.
An acceleration sensor in accordance with the present invention comprises a first piezoelectric plate, a second piezoelectric plate contacting to the first piezoelectric plate, with the polarization axes reversed to each other, thickness of at least one end in the length direction being thicker than that of the rest, a first external electrode provided on the main surface of the first piezoelectric plate, which main surface being a surface opposite to the surface making contact with the second piezoelectric plate, and a second external electrode provided on the main surface of the second piezoelectric plate, which main surface being a surface opposite to the surface making contact with the first piezoelectric plate. A preferred exemplary sensor is that which has a shape of an approximate letter xe2x80x9cLxe2x80x9d, in the cross sectional view along a plane parallel to the length direction of the second piezoelectric plate.
An acceleration device in accordance with the present invention comprises the acceleration sensor of the present invention, a source follower circuit electrically connected with the acceleration sensor, and a stem for mounting the acceleration sensor and the source follower circuit thereon.
With the above described structure, the variance of characteristics is very small because the free vibration section of the biomorph cell and the supporting section, which corresponds to the protruding section of the conventional section, is formed as a single piece member. Furthermore, it is not easily breakable even when the thickness goes thinner for increasing the electrostatic capacitance. This means that the electrostatic capacitance can be increased easily for the detection of acceleration in the lower frequency.